diff --git a/example_with_targets/schema.graphql b/example_with_targets/schema.graphql
index 3ce0104..13e90b1 100644
--- a/example_with_targets/schema.graphql
+++ b/example_with_targets/schema.graphql
@@ -131,6 +131,12 @@ input FunctionTargetAResult {
   status: Int
 }
 
+input SerializationProbe {
+  optionalValue: Int
+  defaultedValue: Int! = 200
+  requiredValue: Int!
+}
+
 """
 The result of API target B.
 """
diff --git a/example_with_targets/src/tests.rs b/example_with_targets/src/tests.rs
index 4de5b0a..42455d9 100644
--- a/example_with_targets/src/tests.rs
+++ b/example_with_targets/src/tests.rs
@@ -1,4 +1,5 @@
 use super::*;
+use shopify_function::wasm_api::{Context, Serialize};
 use shopify_function::{run_function_with_input, Result};
 
 #[test]
@@ -42,3 +43,49 @@ fn test_target_b() -> Result<()> {
     assert_eq!(result, expected);
     Ok(())
 }
+
+#[test]
+fn test_input_object_serialization_omits_none_fields_and_keeps_required_fields() -> Result<()> {
+    let result = serialize_to_json(&crate::schema::SerializationProbe {
+        optional_value: None,
+        defaulted_value: None,
+        required_value: 1,
+    })?;
+
+    assert_eq!(serde_json::json!({ "requiredValue": 1 }), result);
+    Ok(())
+}
+
+#[test]
+fn test_input_object_serialization_includes_some_fields() -> Result<()> {
+    let result = serialize_to_json(&crate::schema::SerializationProbe {
+        optional_value: Some(200),
+        defaulted_value: Some(201),
+        required_value: 1,
+    })?;
+
+    assert_eq!(
+        serde_json::json!({ "optionalValue": 200, "defaultedValue": 201, "requiredValue": 1 }),
+        result
+    );
+    Ok(())
+}
+
+#[test]
+fn test_one_of_input_object_serialization_writes_active_variant() -> Result<()> {
+    let result = serialize_to_json(&crate::schema::Operation::DoThis(crate::schema::This {
+        this_field: "this field".to_string(),
+    }))?;
+
+    assert_eq!(
+        serde_json::json!({ "doThis": { "thisField": "this field" } }),
+        result
+    );
+    Ok(())
+}
+
+fn serialize_to_json<T: Serialize + ?Sized>(value: &T) -> Result<serde_json::Value> {
+    let mut context = Context::new_with_input(serde_json::json!({}));
+    value.serialize(&mut context)?;
+    Ok(context.finalize_output_and_return()?)
+}
diff --git a/shopify_function_macro/src/lib.rs b/shopify_function_macro/src/lib.rs
index d959fa4..52ac74d 100644
--- a/shopify_function_macro/src/lib.rs
+++ b/shopify_function_macro/src/lib.rs
@@ -500,28 +500,53 @@ impl CodeGenerator for ShopifyFunctionCodeGenerator {
                 let field_name_ident = names::field_ident(ivd.name());
                 let field_name_lit_str = syn::LitStr::new(ivd.name(), Span::mixed_site());
 
-                vec![
-                    parse_quote! {
-                        context.write_utf8_str(#field_name_lit_str)?;
-                    },
-                    parse_quote! {
-                        self.#field_name_ident.serialize(context)?;
-                    },
-                ]
+                if ivd.is_required() {
+                    vec![
+                        parse_quote! {
+                            context.write_utf8_str(#field_name_lit_str)?;
+                        },
+                        parse_quote! {
+                            self.#field_name_ident.serialize(context)?;
+                        },
+                    ]
+                } else {
+                    vec![parse_quote! {
+                        if let ::std::option::Option::Some(value) = &self.#field_name_ident {
+                            context.write_utf8_str(#field_name_lit_str)?;
+                            value.serialize(context)?;
+                        }
+                    }]
+                }
             })
             .collect();
 
-        let num_fields = input_object_type_definition.input_field_definitions().len();
+        let num_required_fields = input_object_type_definition
+            .input_field_definitions()
+            .iter()
+            .filter(|ivd| ivd.is_required())
+            .count();
+
+        let optional_field_count_terms: Vec<syn::Expr> = input_object_type_definition
+            .input_field_definitions()
+            .iter()
+            .filter(|ivd| !ivd.is_required())
+            .map(|ivd| {
+                let field_name_ident = names::field_ident(ivd.name());
+                parse_quote! { ::std::primitive::usize::from(self.#field_name_ident.is_some()) }
+            })
+            .collect();
 
         let serialize_impl = parse_quote! {
             impl shopify_function::wasm_api::Serialize for #name_ident {
                 fn serialize(&self, context: &mut shopify_function::wasm_api::Context) -> ::std::result::Result<(), shopify_function::wasm_api::write::Error> {
+                    let field_count: ::std::primitive::usize = #num_required_fields #(+ #optional_field_count_terms)*;
+
                     context.write_object(
                         |context| {
                             #(#field_statements)*
                             ::std::result::Result::Ok(())
                         },
-                        #num_fields,
+                        field_count,
                     )
                 }
             }